Method of making an underwater projectile

ABSTRACT

An efficient underwater projectile is disclosed having a frustoconical nose end, a long thin shank and a shroudless, stepped parallel side wall finned rear section with a forward facing abrupt shoulder formed thereon between two parallel longitudinal edge surfaces, the forward edge surface terminating at a sloped edge surface connecting with the shank, and the fins being joined at the shank body by a rearwardly tapered body section with triangular tapered and rearwardly inwardly sloping outer separation surfaces between the fins.

United States Patent Monson et al.

[ 1 Nov. 12, 1974 METHOD OF MAKING AN UNDERWATER PROJECTILE [75]Inventors: Franklin A. Monson, Glen Arm;

Kenneth E. Mueller, Baltimore, both of Md.

[73] Assignee: AAI Corporation, Corkeysville, Md. 22] Filed: Dec. 24,1970 [21] Appl. No.: 101,431

Related US. Application Data [62] Division of Ser. No. 734,291, June 4,1968.

[52] US. Cl. 29/12 [51] Int. Cl B21k 21/06 [58] Field of Search 29/12,1.22

[56] References Cited UNITED STATES PATENTS 1,131,973 3/1915 White29/1.2

2/1919 Hawkins 29/12 8/1944 Bardell ..29/1.2

Primary E.taminer-Francis S. Husar Assistant Examiner-Leon GildenAttorney, Agent. or FirmReginald F. Pippin, Jr.

[57] ABSTRACT An efficient underwater projectile is disclosed having afrustoconical nose end, a long thin shank and a shroudless, steppedparallel side wall finned rear section with a forward facing abruptshoulder formed thereon between two parallel longitudinal edge surfaces,the forward edge surface terminating at a sloped edge surface connectingwith the shank, and the fins being joined at the shank body by arearwardly tapered body section with triangular tapered and rearwardlyinwardly sloping outer separation surfaces between the fins.

1 Claim, 5 Drawing Figures In the prior art, various projectilearrangements have been employed for underwater ammunition, the mostcommon being spear gun projectiles and shot shell power heads, as wellas combined shot shell and spear projectiles. Such prior configurationshave been substantially less than fully effective, due to suchdeficiencies as cumbersome size, limited velocity, limited effectiverange and small terminal energy for target penetra- Still other objects,features and attendant advantages will become apparent to one skilled inthe art from a reading of the following detailed description of a singlepreferred embodiment constructed according to the invention, taken inconjunction with the accompanying drawings wherein:

FIGS. 1 and 2 are perspective views of a flechett projectile accordingto the invention.

FIG. 3 is a fragmentary longitudinal side view of the rear sectionprojectile of FIGS. 1 and 2, showing its stabilizing fin construction.

FIG. 4 is a rear end view of the underwater projectile of FIGS. 1 and 2.

FIG. 5 is a viewof the projectile during manufacture, and showing thefins in full line prior to final formation, and the broken line in thefinal configuration of FIG. 3.

Referring now in detail to the Figures of the drawings, the projectile11 includes a cylindrical shaft or shank 13 having a frusto-conical nosesection 15 with a flat tip end 15a, and a stabilizing tail sectionincluding canted stabilizing fins 17 each having parallel opposite sidewalls 17k, 17k.

The overall length B of the fins 17 is approximately four shaftdiameters and the overall length A of the frusto-conical nose section 15is approximately three and one-half shaft diameters.

The four stabilizing radial fins 17 are integrally formed with the shank13, as by hot or cold metal working of the shank body to form thedesired configuration, as through the medium of a suitable forming die,to an initial configuration as shown in full lines in FIG. 5, with thecurved outer edge 17f of the fins resulting from the amount of materialdisposed from the shank cylindrical blank into the fin formations whenthe die is pressed in to form the substantially flat or concavetriangular rearwardly and inwardly tapered connecting surfaces 17gbetween'the fins l7t. Surface grinding of t the radially outer surfaceof the fins is employed to effect the desired intermediate and rearwardradial surface final configuration as shown at 17a, 17b, 17c in FIGS. 3and 5. Each of the respective parallel-walled fins is preferably cantedat an angle of approximately 2 with respect to the longitudinal axis ofthe shank 13. It will be appreciated that the canted fins l7 contributeto trajectory accuracy by imparting spin to the projectile, whereby fora given yaw angle the canted fin projectile dispersion is helical andhas a smaller radius of error dispersion than that which would beobtained by a straight finned configuration with a uni-directionaldispersion. The fins 17 are formed by the initial die forming step withan inclined forward face 17d. The intermediate forward-reduced span ordiameter section having a radially outer surface 17a of substantiallyconstant diameter and the enlarged span or diameter rear section havingits radially outer surface 170 also of sub stantially constant spanalong its length areinitially die formed and then surface ground asdiscussed above. At

the longitudinal junction of reduced span section 17a and enlargeddiameter rear section 170 of each fin is radially extending forwardlyfacing flat surfaced shoulder 17b which is also formed by surfacegrinding, preferably during the grinding of the radially outer surfaces17a and 17c. The diameter of the larger diameter rear constant diametersurface 17c substantially equal to approximately one and one-fourth toone and two-fifths diameters of the shaft or shank l3, and thesmaller'diameter intermediate surface is approximately one and one-tenthto one and one-fifth shank diameters, the depth of the shoulder 17bbeing approximately onetenth to one-fifth shank diameters.

The cone angle of the frusto-conical nose section 15 is 10 (:1),included, and the flat tip end 15a of the frustum is approximately .30to .35 of the diameter of the shaft or shank 13. V

The overall length of the projectile is equal to between 30 and 43 shaftdiameters, with a preferred length of 43 diameters for minimum desiredmuzzle velocity of approximately 700 to 750 feet per second andassociated launching propellant chamber pressures.

In one illustrative and preferred embodiment in which the shaft diameteris .100 inch, the shoulder has a depth of .010 inch, and the outerdiameterof the rear fin surface 170 is .131 inch. Also, in thisparticular preferred embodiment, the projectile shank 13 is formed oftungsten, the overall weight being grains, and the overall length being4.3 inches, with a frustrum cone angle of 10 and a tip diameter of .030inches.

In operation, the launching velocity of the projectile is preferablynear that of atmospheric pistol ammunition, being approximately 700 to750 feet per second, and whereas drag forces render atmospheric bulletshapes and other fiechette shapes ineffective in very short ranges underwater, the present projectile is far superior in its range capabilityunder water. The high mass per frontala'rea and specific nose and tailproportions contribute mutually to an efficiently low coefficient ofdrag, and the stability of the projectile flight is also maximized bythe same balance of proportions.

Prior copending application Ser. No. 650,374 filed June 30, I967illustrates an ammunition arrangement through the medium of whichsimilar, but shroudless shouldered finned underwater projectile islaunched. A similar ammunition arrangement may be employed for launchingthe shroudless, shouldered finned underwater projectile according to thepresent invention.

In operation, as the projectile flies through the water the flatfrustrum tip area 1521 separates a bulk of water normal to thetrajectory, creating a cavitation envelope of specific shape and lengthrelative to the instant projectile velocity and depth-The instantaneouscavitation envelope for velocities up to approximately 700 feet persecond and depths up to 90 feet has an effective length extending wellback of the rear finned section 17 of the projectile 11, and may be of alength up to several times the projectile length, dependent upon depthand instant velocity, before pressure can close the cavity. The cavityis difficult to precisely measure and define in structural contents;however, evidence indicates that the forward area of the envelopesurrounding the projectile body is composed of water vapor of varyingdensity levels, the density gradient increasing at a high rate of changefrom the zone immediately adjacent the projectile body to the zone atthe edge of the cavitation envelope, and the lines of equal vapordensity extending in something of an arcuate form from the zone of theflat tip a in a generally convex form along the length of the projectileand the remainder of the envelope rearwardly to the zone of closurewhere larger water droplets are forced into the envelope zone. It hasbeen found that the density gradient of the water vapor increases sorapidly from the zone adjacent the projectile body that small increasesof tail fin diameter or projectile length, with respect to shaftdiameter, cause very large increases in drag with resultant decrease invelocity and effective range and terminal energy of the projectile. Thetail configuration 17 raises a stabilizing force from collision with thevapor within the cavitation envelope, and as the water vapor thus exertsa drag force on the tail section at the same time, it will beappreciated that the tail section must be formed with the concept ofminimizing contact with water vapor while insuring sufficientstabilizing force to accomplish stability or projectile flight. With theforegoing mentioned configuration and relative dimensions and weights,for muzzle velocities of approximately 700 feet persecond and depths upto 90 feet, the projectile is nearly free of boundary layer drag forcesover its effective range except for the area of the frustrum flap tipend 15a, and the very minute drag force exerted on the tail section 17by the low density water vapor in the central zone of the cavitationenvelope cross section. The area of the flat tip end 15a is optimizedfor minimum drag and desired cavitational envelope vapor impingement forstability control on the tail section 17, a frustrum tip end 15a ofsmaller proportion producing erratic flight with severe yaw resulting innegligible effective range, whereas increase in the proportions of theflat tip end 15a results in higher drag forces at this zone, which alsodecreases the effective range. In addition, it has been found thatincreases in the cone angle of the nose section 15, and/or the outerdiameter of the fins 17 also results in higher drag forces whichdecrease the effective range, as will be apparent from considerations ofthe rapid radial increase in vapor density of the water vapor within thecavitation envelope. While use of a shroud as disclosed in applicationSer. No. 650,374 effectively minimizes yawing and increases flightaccuracy of the projectile the shroud'also reduces velocity materiallymore than the present invention, particularly due to the inherentlylarger frontal and surface area of the shroud and the drag resultingfrom axially vectored impingement of the water vapor particles on thislarge frontal and outer surface area. The present intermediate shouldersl7b between thetwo constant diameter surfaces 17a, 17c on the fins 17have surprisingly been found to be substantially equivalent in reducingand have resulted in further increased accuracy, apparently due largelyto the materially less drag, and higher terminal velocities or longereffective range for a given initial velocity than that of the priorshrouded finned projectile. Further, while the shrouded projectile ofSer. No. 650,374 is quite satisfactory in shallow depths, this shroudedprojectile has inadvertently decreased accuracy as water depthincreases, and the present shroudless stepped shouldered. finnedprojectile has been found to afford materially better accuracy atincreased depths than the shrouded projectile, due to its decreasedprior shrouded finned projectile.

It has further been foundthat reduction of the overall length relativeto diameter and tip end area results in less momentum with proportionateloss in effective range, unless the launching velocity is increasedinversely proportionate to the length reduction. In this respect, thepreferred length of the overall projectile 11 has beenfound'to be 43diameters, and by a sacrifice of higher launching velocities andassociated higher chamber pressures the length of the projectile hasbeen successfully varied down to a length of 30 shaft diameters. Itwill, of course, be appreciated that such requirement for higherlaunching velocities and associated higher firing chamber pressures forlaunching the projectile'put severe requirements upon the weight andstrength of the launching ammunition arrangement and/or barrel withconsequent increase inbulkiness and the weight of the equipment anddecrease in efficiency of power utilization.

It has been found that the terminal efficiency of the remainingenergywhen the projectile is at a 30 foot range, which is the generalvisibility range in shallow water of up to 3 foot depth, equals normallybetter than approximately one-half that of the energy of the projectileat the launch location, and produces penetration force concentration ofan extreme degree over a distance equal to and beyond this 30 footvisibility range.

- At the 90 foot depth the terminal efficiency of the remaining energyis about 25 percent of that of the launch location energy of theprojectile, with consequent lesser but effective penetration force.

It will accordingly be appreciated that the critical interrelation ofthe various parameters of the projectile are effective overall toprovide a highly desirable and useful underwater projectile which may beeffectively used by underwater fishermen, divers, etc., with goodresults within the operating ranges of visibility in water depths up tofeet, and using launching velocities conventional to atmospheric pistolammunition in the vicnity of 700 feet per second.

While the invention has been described with respect to a singleillustrative and preferred embodiment, it will be apparent to thoseskilled in the art that various modifications may be made withoutdeparting from the scope and spirit of the invention. Accordingly it isto be understood that the invention is not to be limited by theillustrative embodiment, but only by the scope of the appended Claims.

That which is claimed is:

l. The method of making an underwater projectile comprising:

forming a cylindrical shaft having opposite flat ends,

surface grinding the radial surfaces of the fins to a firstsubstantially constant radius at all longitudinal points along arearward section of said fins,

and surface grinding the radial surfaces of the fins to a second smallerradius substantially constant along a forward section of said fins whilegrinding a forwardly facing shoulder on each of said fins and adjoiningsaid rearward and forward different constant radii sections.

1. The method of making an underwater projectile comprising: forming acylindrical shaft having opposite flat ends, taper-grinding at one endof said shaft to a degree sufficient to leave a flat blunt end wherebysaid one end is frustoconical and flat-ended, impact die forming theopposite end of said cylindrical shaft by impact in a forming die andthereby working the cylindrical shaft material into a convex finnedshape with the fins extending radially beyond the shaft diameter withthe connecting shaft body being rearwardly inwardly tapered from thebeginning to the ends of the fins, surface grinding the radial surfacesof the fins to a first substantially constant radius at all longitudinalpoints along a rearward section of said fins, and surface grinding theradial surfaces of the fins to a second smaller radius subsTantiallyconstant along a forward section of said fins while grinding a forwardlyfacing shoulder on each of said fins and adjoining said rearward andforward different constant radii sections.